1. Field of the Invention
This invention relates, in general, to automatic gain control circuits and more particularly to a circuit that detects the onset of, and prevents saturation and cut off of an amplifying transistor of a speech network.
2. Background Art
An Electronic Telephone Chip (ETC) typically comprises a dual tone multifrequency dialer, a speech network, a microprocessor, and a voltage regulator. The low voltage, i.e., 1.1 volts, at which the ETC operates, presents inherent problems in these circuits. Furthermore, the problem is magnified as the number of telephones on a line increases. If the transistor amplifier in the speech network saturates on positive peaks of the voice signal or cuts off on negative peaks, a harsh sound or crackle is typically emitted from the phone.
To remedy this problem, typically, a multitransistor circuit is provided for draining excess signal current from the amplifying transistor. A pair of differentially connected PNP transistors will draw excess current from the collector of the NPN amplifying transistor and activate switching transistors that draw current from the base of the NPN amplifying transistor, thereby limiting the positive peak of the incoming signal.
A similar arrangement comprising a pair of differentially connected NPN transistors and a number of switching transistors would limit the negative peaks. Such a configuration consumes excessive chip area and increases cost.
Therefore, what is needed is an automatic gain control circuit having reduced chip area consumption that detects the onset of saturation and cut-off and limits the positive and negative peaks of an incoming signal by preventing an NPN transistor amplifier from going into cut-off or saturation.